1. Field of the Invention
This invention relates to a method for producing a remarkably efficient peptidase using a koji (yeast) mold. More particularly, it is concerned with a method which comprises culturing a koji mold in a nutrient culture medium containing one or more fatty acids having 14, 16, 18, or 20 carbon atoms, their derivatives, or a vegetable oil containing these fatty acids, and recovering the accumulated peptidase from the culture medium.
2. Discussion of the Prior Art
Koji mold has been utilized in Japan for centuries in brewing shoyu (Japanese soy sauce), Japanese sake, or bean paste, and it is well known that peptidase enzymes produced by these koji molds play a central role in these brewing processes. One of the most serious problems of shoyu brewing is that it is a time-consuming process; usually it takes 6 to 12 months.
On the other hand, a chemical soy sauce has been manufactured by a process which includes an acid-catalyzed hydrolysis of defatted soybeans. This process was carried out under intensive conditions using a large amount of conc. hydrochloric acid at a high temperature (100.degree.-120.degree. C.) for more than 6 hours. However, in the acid-catalyzed hydrolysis process, although the hydrolysis ratio of defatted soybean is extremely high and the soybean is hydrolyzed completely, the chemical soy sauce so obtained is inferior to brewed shoyu in quality. Besides, useful substances unstable to acid hydrolysis such as tryptophan and carbohydrates contained in defatted soybean are decomposed to form undesirable substances such as disagreeable organic acids and colored material.
Recently, an enzyme-catalyzed hydrolysis process (enzyme process) has been tried to manufacture a shoyu within as short a time as 2 to 4 weeks. This process comprises hydrolyzing a defatted soybean with enzymes obtainable from koji mold, and attempting to impart to the resultant hydrolyzate a preferable shoyu-like flavor by lactic and yeast fermentations. However, it is pointed out that the shoyu-like seasoning obtained by the modified enzyme process is inferior to shoyu in taste, as well as flavor, because of a low hydrolysis ratio of protein to amino acids, and especially because of a low liberating ratio of glutamic acid.
Although an enzyme-catalyzed hydrolysis of pure proteins such as milk casein or fibroin might be expected to produce an aqueous solution of amino acids suitable for medical or nutritional use, the production of such an aqueous solution of amino acids has never been adequately realized. One of the reasons heretofore preventing the practical use of such hydrolyzates undoubtedly resides in the fact that the extent of protein hydrolysis is insufficient. To be useful for this purpose, a protein has to be hydrolyzed to its constituent amino acids as completely as it is in an acid-catalyzed hydrolysis process.
A need therefore continues to exist for a method of producing an efficient peptidase preparation which is capable of hydrolyzing protein almost completely to its constituent amino acids.